This invention relates to continuously operating centrifuges for separating solids from fluid mixtures, for example, sugar centrifuges. The invention is particularly directed to a centrifuge of the type having a frusto-conical drum rotatable about a vertical axis and having its larger diameter end facing upwardly.
In centrifuges of this type, a dished feed member is generally provided at the lower end of the frusto-conical drum, and a frusto-conical cover screen supported by a frusto-conical supporting basket is mounted on the inside of the drum. Means are provided for directing a mixture to be separated to the vicinity of the dish shaped feed member, so that the material can pass onto the cover screen. The cover screen is rotatable with the drum, so that the liquids to be separated from the material pass through the cover screen in successive zones until it reaches the upper end of the drum. The solids may be removed after passing upwardly along the inner surface of the cover screen.
The above described continuously operating centrifuges have been employed for a considerable period of time, and their manner of operation and construction are well known. In practical application, and in particular in the case of sugar centrifuges, the particle sizes of the solids which must be separated from the liquid varies over a very large range within the mixture. The cover screen is designed, with respect to the dimensions of the circular or slot-shaped holes therein, so that either the solids of all particle sizes are substantially held back by the cover screen, or so that only the coarser particles are retained by the screen. In the latter case, the fine particles are discharged together with the liquid run-off (e.g., molasses), and a further separation process is necessary to remove the fine particles from the liquid. When the holes in the cover screen are selected to pass the fine solids, the separating performance, and hence the throughput, of the centrifuge is high, even though a substantial portion of the fine particles of the solids is lost. While the holes in the cover screen may be designed to also retain the fine particles, as discussed above, in order to prevent the loss of the fine particles, this expedient has the disadvantage of a considerable reduction in the separating performance of the centrifuge, and hence a reduction in the throughput of the centrifuge.